The present invention relates generally to industrial controllers and, in particular, to current-loop circuits used to connect industrial controllers to controlled equipment and processes.
Two wire, current-loop circuits are used to provide electrical signals to a variety of industrial devices, for example, valve actuators or meters. Such circuits typically produce a defined current output, generally having an on state from 4-20 mA, according to a received control signal. By controlling output current instead of output voltage, changes in resistance caused by different lengths of wire connecting the current-loop circuit to the load and variations in load resistance, are better accommodated.
A typical current-loop circuit may use a current mirror circuit providing a controllable current source that generates the 4-20 mA signal based on the control signal. The current source is supplied with power from a voltage supply, commonly referred to as the compliance voltage, having a voltage value sufficient to drive the peak current required across the range of expected loads. For example, the loads may range in resistance from approximately 750 ohms for a solenoid valve to approximately 0 ohms for a panel meter. To provide adequate range of currents for these different loads, typical loop driving circuits are provided with compliance voltage sources of at least 24 volts.
While the original current-loop circuits operated in a binary mode, current-loops are also used to provide for “analog” control of current providing any current within a predetermined range of currents.
The power dissipated by a current source used in a current-loop driver will depend on the excess compliance voltage beyond the voltage needed to provide the desired current output. Thus, while it is desirable to have a high compliance voltage to provide high current outputs to high resistance loads, such high compliance voltages can produce high power dissipation in the current drive circuits when lower currents are output or lowered resistance loads are used.
One solution to this dilemma is to provide the current source, typically a transistor that is powered by a multi-mode power supply providing one or more different compliance voltages or a continuous range of compliance voltages. The transistor provides rapid current control and the power supply is switched between voltages at a slower rate depending on the amount of excess compliance voltage for the given current that is required. U.S. patent application 2006/0066379, filed Mar. 30, 2006, assigned to the assignee of the present invention and hereby incorporated by reference, describes such a system. In this system, the power supply is a boost converter, operating with very low power dissipation, to provide a range of compliance voltages to a field-effect transistor (FET) that provides the current control. The particular compliance voltage level to be used is determined by comparing the voltage dropped across the FET against the voltage drop across the load so that the compliance voltage may be tailored to the particular resistance of the load and the desired current level.